In the semiconductor manufacturing world, manufacturers produce plasma processing chambers that utilize radio frequency (RF) power to generate a plasma. In order to achieve efficient power transfer between the RF generator (“generator”) and the plasma load, an impedance-matching network (“match network”) is often used to match the load impedance to a desired input impedance, typically 50 ohm. Plasma load impedance may vary depending on variables such as generator frequency, power, chamber pressure, gas composition, and plasma ignition. The match network accounts for these variations in load impedance by varying electrical elements, typically vacuum variable capacitors, internal to the match to maintain the desired input impedance.
FIG. 1 illustrates a typical generator, match network, and plasma load system. The generator 102 transmits RF power to the match network 104 via a transmission line 108 (e.g., coaxial cable) and then onto the plasma load 106 via an electrical connection 110. The match network 104 varies its internal electrical elements such that the input impedance of the match network 104 is close to the desired input impedance. Match networks typically only contain reactance elements, meaning elements that store energy in electrical and magnetic fields as opposed to resistive elements that dissipate electrical power. The most common reactance elements are capacitors, inductors and coupled inductors but others such as distributed circuits are also used. Match networks can also include lossless elements including transmission lines and transformers. The only resistive elements in a match network are typically associated with losses in non-ideal reactive and lossless components or components that do not take part in the impedance transformation such as components for sensing voltage, current, power or temperature.
Match networks can comprise a number of variable reactance elements. To match a load impedance that can vary over a certain impedance range to a desired input impedance, the prior art typically uses at least two variable reactance elements, or a combination of a variable generator frequency and a single variable reactance element. Alternatively, if a certain input impedance mismatch can be tolerated, a single variable reactance element in combination with a generator having a fixed frequency or a fixed match with a variable frequency generator may be used. The variable reactance elements are often variable capacitors, variable inductors, or a combination of the two. For instance, sets of switches, capacitors, and inductors can be used to form a match network. Vacuum variable capacitors are one example of a variable reactance element. Variable capacitors can be arranged between two terminals via parallel connections of fixed capacitors selectively shorted to the second terminal via a switch. Capacitance is thus altered by switching one or more of the switches thus varying the effective capacitance between the two terminals.
FIG. 2 illustrates one embodiment of a match network comprising one switched variable capacitor circuit. The switched variable capacitor 200 is formed via a group of fixed capacitors, the first indicated by 220 and the last by 222. The switched variable capacitor 200 typically contains between one and one hundred fixed capacitors all connected to a first terminal 202 and selectively connected to a second terminal 204. Switches, of which the first is indicated by 230 and the last by 232, selectively control which fixed capacitors are connected to the second terminal 204. Varying the number of fixed capacitors connected to the second terminal 204 varies the net effective capacitance of the switched capacitor 200. To match the plasma load 106 impedance to a desired input impedance the match network 104 also contains a fixed inductor 210 and a second variable capacitor 212 which may for example be of the vacuum variable type.
One example of switches 230, 232 is a PIN diode. PIN diodes are PN diodes with a lightly-doped intrinsic semiconductor region between the p and n-doped regions. PIN diodes have been used as switches in match network variable capacitors, because they have low losses in both the on and off-state, can handle high current in the on-state, and can handle high voltage in the off-state. PIN diodes achieve these characteristics by virtue of their unique operation at RF frequencies. In the off-state, the intrinsic region is largely devoid of carriers and this along with its large width gives the intrinsic region high resistance. As a result, the intrinsic region is loath to pass direct current (DC) and thus has low DC leakage current. Similarly, the intrinsic region bounded by the charges in the doped regions acts as a low capacitance capacitor, thus presenting a high impedance to alternating current (AC). The large width of the intrinsic region also allows the PIN diode to withstand high voltages in the off-state.
In the on-state, a PIN diode is forward biased, and holes from the p-region and electrons from the n-region are injected into the intrinsic region. Due to the long carrier lifetime in the intrinsic region many of these carriers do not recombine even if a reverse voltage is applied for a sufficiently short period of time and thus they make the intrinsic region highly conductive to AC with sufficiently high frequency. Hence, the PIN diode has very low losses in the on-state when AC with sufficiently high frequency is applied. This conductivity increases as greater DC bias is applied—as more carriers are injected into the intrinsic region. Further, the carrier lifetime in the intrinsic region is longer than the RF cycle, so rather than being swept out of the intrinsic region, carriers are rocked back and forth within it via the RF field. This property allows the PIN diode to see very little losses when RF current passes through it in the on-state.
However, PIN diodes are very expensive and only have two terminals. Thus, the RF current and the DC control current must enter via the same terminal requiring complicated, expensive, and bulky circuitry (e.g., inductors) to isolate the DC control source from the RF source.